Miniaturization is always required in the design of an RF circuit of a cellular phone. In recent years, the cellular phone is required to implement various functions and, to this end, it is preferable to incorporate as many components as possible in the device. However, there is a limitation in the size of the cellular phone, which makes it difficult to reduce the occupying area (mounting area) and height dimension of the RF circuit within the cellular phone. Thus, the components constituting the RF circuit are required to be small in terms of the occupying area and height dimension.
Under such circumstances, a thin-film piezoelectric filter formed using a thin-film piezoelectric resonator which is compact and capable of reducing weight has come to be utilized as a band-pass filter used in the RF circuit. The thin-film piezoelectric filter as mentioned above is an RF filter using a thin-film piezoelectric resonator (Thin Film Bulk Acoustic Resonator: FBAR) having a structure in which a piezoelectric thin-film made of aluminum nitride (AlN), zinc oxide (ZnO) or the like is formed on a semiconductor substrate in a sandwiched manner between top and bottom electrodes, and a cavity is formed immediately under the piezoelectric thin-film so as to prevent an elastic wave energy from propagating into the semiconductor substrate.
FIGS. 17A to 17C illustrate an example of a conventional thin-film piezoelectric resonator. FIG. 17A is a schematic plan view of the conventional thin-film piezoelectric resonator, FIG. 17B is a cross-sectional view taken along X-X line of FIG. 17A, and FIG. 17C is a cross-sectional view taken along Y-Y line of FIG. 17A. The thin-film piezoelectric resonator illustrated in FIGS. 17A to 17C has a substrate 6 having an air gap 4 thereon and a piezoelectric resonator stack 12 suspended from the substrate 6 with the peripheral portion thereof supported by an edge portion on the top surface of the substrate 6 near the air gap 4. The piezoelectric resonator stack 12 has a piezoelectric thin-film 2 and bottom and top electrodes 8 and 10 formed so as to sandwich the piezoelectric thin-film 2. Hereinafter, layers of the piezoelectric thin-film, bottom electrode, and top electrode are sometimes referred to as a piezoelectric body layer (piezoelectric layer), a bottom electrode layer, and a top electrode layer, respectively.
The piezoelectric resonator stack 12 is a laminated body of the piezoelectric layer 2, bottom electrode layer 8, and top electrode layer 10 and is suspended at the peripheral portion thereof, and both main surfaces in the center portion (portion corresponding to the air gap 4) contact a surrounding gas such as air, or vacuum. In this case, the piezoelectric resonator stack 12 forms an acoustic wave resonator having a high Q-value. An AC signal added to the bottom electrode layer 8 and top electrode layer 10 has a frequency equal to a value obtained by dividing the acoustic velocity in the piezoelectric resonator stack 12 by twice the weighted thickness of the piezoelectric resonator stack 12. That is, in the case where fr=v/2t0 (fr is resonance frequency, v is acoustic velocity in the piezoelectric resonator stack 12, and t0 is weighted thickness of the piezoelectric resonator stack 12) is satisfied, the piezoelectric resonator stack 12 resonates by the AC signal. Since the acoustic velocity in each layer constituting the piezoelectric resonator stack 12 changes for each material constituting each layer, the resonance frequency of the piezoelectric resonator stack 12 is determined not by the physical thickness of the stack 12 but by the weighted thickness calculated by taking into account the acoustic velocities of the piezoelectric layer 2, bottom electrode layer 8, and top electrode layer 10 and their physical thicknesses. A vibration region where the resonance of the piezoelectric resonator stack 12 occurs is a region where the top electrode 10 and bottom electrode 8 overlap each other when viewed in the thickness direction.
It is known that characteristic degradation occurs due to a lateral acoustic mode in a conventional thin-film piezoelectric resonator in the case where the resonator is formed into a quadrangle or circle.
PTL 1 discloses a technique for preventing occurrence of the characteristic degradation due to the unnecessary lateral acoustic mode (spurious vibration). FIGS. 18A and 18B each illustrate a cross-sectional view of a thin-film piezoelectric resonator disclosed in PTL 1. In this technique, a frame-like zone 60 is provided at the end portion (peripheral portion) of the top electrode so as to prevent occurrence of noise caused due to the lateral acoustic mode. FIG. 18A illustrates a structure adopted in the case where the piezoelectric layer is made of a piezoelectric material of type 1, such as ZnO, having a dispersion curve of a low frequency cutoff type, and FIG. 18B illustrates a structure adopted in the case where the piezoelectric layer is made of a piezoelectric material of type 2, such as AlN, having a dispersion curve of a high frequency cutoff type.
Further, increases in a quality factor (Q-value) and an electromechanical coupling factor (kt2) are required as important characteristics of the thin-film piezoelectric resonator. When the Q-value is increased, the insertion loss of an FBAR filter can be reduced, so that the increase in the Q-value is a very important factor for the thin-film piezoelectric resonator. Further, the kt2 is a factor determining the frequency interval between the resonance frequency of the thin-film piezoelectric resonator and its antiresonance frequency. When the kt2 is increased, the passband width of the FBAR filter can be widened.
FIGS. 19A and 19B illustrate an example of an impedance characteristic diagram of a thin-film piezoelectric resonator and an example of a Smith chart of a thin-film piezoelectric resonator, respectively. An impedance (Rs) and Q-value (Qs) at a resonance frequency (fs) and an impedance (Rp) and Q-value (Qp) at an antiresonance frequency (fp) are main characteristic parameters. In order to increase the Q-value at the resonance frequency fs and antiresonance frequency fp, Rs is needs to be reduced and Rp needs to be increased. In the Smith chart of FIG. 19B, the left end of the chart indicates the resonance frequency (fs) and right end indicates the antiresonance frequency (fp). In the frequency band (upper half of the from the fs to fp, a curve closer to the outer circumference of the chart exhibits better characteristics of the thin-film piezoelectric resonator. In the thin-film piezoelectric resonator, the Rs depends greatly on the electric resistance of an electrode, and the Rp depends greatly on the thermal loss of elastic energy and energy loss caused due to propagation of an elastic wave energy to outside the vibration region.
PTL 2 describes that an AlN thin-film is used to introduce a structure having an top electrode with an increased thickness on the frame formed at the outer periphery of the vibration region and, thereby, a thin-film piezoelectric resonator capable of suppressing occurrence of the spurious vibration and excellent in the Q-value can be obtained.
PTL 3 discloses a thin-film piezoelectric resonator having an annulus located on the surface of one of top and bottom electrodes. A region inside the annulus has a first acoustic impedance, the annulus has a second acoustic impedance, and a region outside the annulus has a third acoustic impedance. The second acoustic impedance is higher than the first and third acoustic impedances. PTL 3 describes that, with the above configuration, it is possible to obtain a thin-film piezoelectric resonator excellent in the Q-value.
The piezoelectric resonator stack constituted by the piezoelectric layer, bottom electrode, and top electrode is formed above the cavity, so that it is fragile in structure and is subject to mechanical breakage in the production process. Thus, as described in PTL 4 and PTL 5, in order to prevent the breakage of the thin-film piezoelectric resonator, proposed is a configuration in which the cavity is covered with the bottom electrode, i.e., the bottom electrode is formed on the substrate in contact with the surface of the substrate.
When there exists a region where the top and bottom electrodes overlap each other outside the cavity, an unnecessary capacitance is generated to reduce an effective electromechanical coupling factor (effective kt2). Therefore, it is proposed that the region where the top and bottom electrodes overlap each other be formed within the cavity.